End-to-end speed improvements: (measured on tempete_cif.y4m,
20 frames for encoder and all 260 frames for decoder)

* GLOBAL_MOTION encoder: ~10% faster
* GLOBAL_MOTION decoder: 100-200% faster depending on bitrate
* WARPED_MOTION encoder: ~2.5% faster
* WARPED_MOTION decoder: ~20-40% faster depending on bitrate

The improvement in the GLOBAL_MOTION decoder is particularly
large because its runtime is dominated by calls to warp_plane().

This introduces minor changes to the output of the warp filter,
but these should be rare.

Change-Id: I5813ab9e90311e27587045153c32d400b6b9eb92